Investigators have expended considerable effort in the study of surfaces in space which are selectively passive to the transmission of electromagnetic energy. These surfaces are configured as thin periodic arrays of either slots or dipoles. In consequence of Babinet's principle, the results of theoretical analysis of the former find direct applicability to the latter.
Collectively, periodic arrays of slots or dipoles function as band-filters of electromagnetic radiation. Conceptualized from a circuit standpoint, periodic arrays of dipoles are band-stop, or reflection filters. Within their operating band, properly designed periodic arrays of dipoles reflect incident signals in a manner comparable to a highly conducting solid metal surface. Outside of this reflection band, however, incident signals pass through the array of dipoles. Periodic arrays of slots perform a complementary roll with respect to dipole arrays. For example the periodic slot arrays function as an electromagnetic window within their operating band, i.e. they are band-pass devices permitting the incident electromagnetic signals to pass through the array. Outside of the operating band, such arrays become opaque, serving to reflect the incident signals.
A variety of applications utilizing these space filters have been proposed. For example, a periodic array of dipoles can be employed to replace the solid metal surface for applications in which an extended reflection bandwidth is not needed or may be undesirable. Arrays of crossed dipoles have been employed as a Cassegrain subreflector in a dual-frequency antenna system, while arrays of slots have been applied in the design of radomes, particularly those intended for use with aircraft. Such radomes promise several operational improvements. For example, conventionally structured aircraft radomes, formed of rigid dielectric or ceramic materials, may develop precipitation noise at high speeds and occasioned by static charge buildup and subsequent discharge to the airframe. Such discharge has represented a hindrance to the performance of enclosed equipment. Reflection lobe phenomena are typically encountered in most applications, and as requirements for scan angle flexibility have enlarged, a variety of effects are encountered. For instance, a transmission loss and phase distortion may be witnessed. Further, the equipment enclosed by more conventional radomes is susceptible to lightning damage as well as to thermal problems developed by poorly controlled frictionally induced skin heating.
Metallic radomes promise such advantages as the elimination of precipitation noise, inherent lightning protection, improved shielding against spurrious low frequency pulses due to the above-noted band-pass filter characteristics; and a potentially improved mechanical strength. However, due to aerodynamic design constraints, the geometric shapes which the radomes must assume (for example, ogival or conical) have developed a need to accommodate relatively large scan angles of incidence.
One slotted array structure contemplated for use within aircraft radomes is described in U.S. Pat. No. 3,975,738. See also the following publication.
I. pelton, E. L. and B. A., Munk, "A Streamlined Metallic Radome," IEEE Transactions on Antennas and Propagation, Vol. APA-22 No. 6, Nov. 1974, pp. 799-803.
The slot elements of the array there described are in a general "Y" shape, each slot element being formed as a continuous geometric shape with adjacent outwardly disposed arms being arranged at angles of 120.degree. with respect to each other. The reactive loading achieved with the noted geometry achieves a frequency-stable pass for a broad range of incident signals and accommodates polarization variations. However, the structure, which must be formed by chemical etching, requires a supportive substrate inasmuch as "islands" of conductive or metallic material are situated within each cluster of the noted arms. This feature necessarily poses a limitation upon the strength of any radome utilizing the design and requires the presence of a supporting dielectric substrate. As is apparent, the mechanical integrity of the slot structure is impaired with such an arrangement.
Another structure configured to avoid difficulties encountered due to incidence angle variations is described in U.S. Pat. No. 3,789,404. In this document arrays are described comprising resonant short dipole elements of length less than one half wavelength which are loaded in the manner of a two-wire transmission line. Further, a similar array structure has been utilized to develop a space filter for use as a low loss dichroic plate permitting the simultaneous single antenna transmission of both X and S band energy. Such an arrangement is described in U.S. Pat. No. 3,769,623.